The invention relates to an apparatus for producing a bias voltage for application to a developing electrode of a developing unit contained in an electrophotographic copying machine.
As is recognized, the electrophotographic process comprises the step of exposing a charged surface of a photosensitive member to a light image which is projected through an exposure optical system, thereby selectively removing the surface charge to form an electrostatic latent image thereon. The latent image is converted into a visual image by bringing it into contact with a colored fine particle which is referred to as a toner. The toner is charged to the opposite polarity from the charge which forms the latent image, whereby it is electrostatically attracted thereto. While non-image areas of the latent image should be free from the deposition of any toner as a result of the loss of electric charge thereon, there remains a potential on the order of 200 to 300 volts actually in these areas because of the problem of fatigue of the photosensitive member, and the toner attaching to these areas cause a background smearing or scumming of a resulting copy image. To avoid this, it is known to utilize a developing electrode which is located in close proximity to and along the surface of the photosensitive member and which is applied with a bias potential slightly exceeding that of non-image areas, thus effectively preventing the deposition of any toner to these areas. The potential of the latent image, including non-image areas, depends on the amount of exposure applied. The less the amount of exposure, the higher the potential or vice versa. Thus, the potential of the latent image will be constant for a given amount of exposure assuming that other factors remain constant.
However, as a matter of practice, a light source used in the exposure optical system has a light output which varies with time. By way of example, FIGS. 1 and 2 show some examples the light output characteristics of fluorescent lamps. With a lamp having the characteristic as shown in FIG. 1, the light output from the fluoroescent lamp varies from the initial amount, taken as 100%, to a value between 75 and 80% in a brief time interval of the order of about 10 minutes during which a continued copying operation takes place. FIG. 2 shows the lamp output characteristic over a prolonged period of time, and it will be seen that the output will be similarly reduced to an initial value between 75 and 80% after 100 hours of use. Therefore, it will be seen that the application of a fixed bias voltage to the developing electrode cannot provide a satisfactory assurance against scumming because a variation in the amount of light output from the light source causes a change in the potential of non-image areas. Thus it is seen that it will be desirable to provide a source of bias voltage which is automatically controlled in response to a variation in the amount of light output from a light source used in the exposure optical system. In the prior art practice, there has been no remedy to correct for a variation in the light output with time although a lamp regulator is proposed to accommodate for a fluctuation in the line voltage.